The invention relates to screens, and more particularly to screens used to cover inlets of engines and like apparatus. More specifically, the invention relates to heated screens which are used to protect aircraft engines from damage caused by large foreign bodies in the intake air.
Foreign bodies drawn into an aircraft engine along with the intake air can pose a substantial threat of engine failure. For example, a large bird or another foreign body drawn into an engine together with intake air can produce severe engine damage.
Screens have therefore been provided in engine inlets, to prevent harmfully large foreign bodies from being carried into the engine. The mesh size of the screen is chosen such that foreign bodies which can pass through the screen are unlikely to cause damage.
Screens of this type work well in non-icing conditions, i.e. when water content of the air, temperature, airspeed, etc. are such that ice does not accrete on the aircraft. However, in icing conditions, a screen of this type can itself be a source of foreign body engine damage.
In icing conditions, ice builds up on screens as now constructed. The ice accretion can be substantial enough to seriously restrict air flow. Furthermore, ice chunks may grow until the forces exerted on them by vibration and the intake air break them off the screen. These chunks, which can reach a dangerously large size, may then be drawn into the engine and can damage it. Also, accreted ice may be released from the screen if the aircraft enters warmer air where the temperature is above freezing.
One object of the invention is to provide a screen--for use with engines and like apparatus--on which ice cannot form.
Another object is to provide such a screen which is heated uniformly across its surface.
Still another object is to provide a screen which is rigid enough to stand up in use while remaining ice free.
A further object is to improve on known screens of this type.
In accordance with the invention, a screen is formed of like electrically conductive wires, each extending between two terminals (to which a power source may be connected). The wires cross each other at crossover points. The wires are arranged--i.e. the crossover points are selected--such that when the terminals are connected across a power source (a) all wires which cross a particular crossover point have the same potential at that crossover point and (b) all crossover points along any given wire have different potentials. Thus, when current is passed through the screen, it passes throughout the entire network of wires, with each wire carrying essentially the same current. Accordingly, all parts of the screen are equally heated.
In the preferred embodiment, the wires are uninsulated and are connected at the crossover points, as by spot welding, brazing, etc. This has no electrical consequences; because the wires are at equipotential at the crossover points (even if they do not touch each other at those points), no current flows from one wire to another. However, the connection is preferred because the screen is more rigid with these connections than without them.